1. Field
The following description relates to a method and an apparatus for generating a pair of orthogonal sets with a wide range of spreading factors.
2. Description of Related Art
Orthogonal codes have become an integral part of any direct sequence spread spectrum based communication system for communicating over a wireless channel. Typically, one or more sets of codes are generated and provided to a spread spectrum based communication system for assignment to user(s). Here, each set contains codes, which are orthogonal in nature. The orthogonal nature of the codes ensures that different users can communicate over the same channel with a least multiple access interference (MAI). This allows assignment of single or multiple orthogonal codes in a set to the user(s) in the spread spectrum based communication system depending on a user requirement. However, a total number of orthogonal codes in a set that are deployed in a spread spectrum is limited by a processing gain or a spreading factor of the spread spectrum based communication system.
A number of users supported in a typical spread spectrum based multi-user communication system is usually less than a processing gain/spreading factor, and the system is said to be under-loaded. Next generation communication systems call for a higher user capacity than what could be achieved through a conventional under-loaded system. One efficient way to achieve this is by overloading the system with additional codes such that a total number of codes exceeds the spreading factor. Such a system is referred to as an overloaded or oversaturated system, and the channel is referred to as an overloaded channel. Overloading has been considered as an attractive candidate for next generation applications requiring high capacity. The key challenge in designing an overloaded system is to identify codes that can communicate over a spread spectrum channel with minimal interference.
One important application of overloading is to support more number of users than a spreading factor of a system. This feature is included in Code Division Multiple Access (CDMA) standards whereby the extra users are supported with a graceful degradation in a bit error rate (BER). Another aspect of overloading may be to impart scalability and flexibility in a data rate by assigning multiple spreading codes to users as implemented in Multi-Code CDMA. This feature is also extensively used in a scalable sub-band ultra-wideband (S-SUWB) system and an orthogonal frequency-division multiplexing (OFDM) SUWB system.
These systems present energy efficient means of transmitting an ultra-wideband (UWB) signal using sub-band technology for wireless personal area communication. A significant energy saving in such systems comes from orthogonal code based sub-band selection, which obviates the need of individual down conversion and filtering. An enhanced flexibility of resource allocation for multiuser support is addressed in S-SUWB systems by improvising upon scalability in a data rate, a quality of service (QoS), and multiuser support in both uplink and downlink communication. This provision calls for a plurality of orthogonal codes in a transceiver.
A common approach used to generate a pair of orthogonal codesets for overloading employs a scrambling operation. An existing orthogonal set is selected as a first codeset consisting of N codes. Additional M codes of a second codeset are obtained by scrambling the orthogonal set with a random scrambling sequence. The scrambling strategies almost always results in codes with more or less similar cross-correlation values with one another, thereby rendering uniform performance to all the codes. This aspect is favorable only when the system overloading is 100%, which is often not the case in practical systems. Moreover, the majority of existing art addresses a spreading factor of a form N=2n.
Due to abovementioned reasons, it is evident that the existing system does not provide zero interference transmission in synchronous spread spectrum systems using orthogonal codes in overloaded systems since a total number of available orthogonal codes is limited to a spreading factor. There is a need for a method to employ diverse spreading factors codes such that a mutual interference is as low as possible.